T
he first time I dissected a black marlin weighing
more than 500kg is something I will never
forget. It was on Lizard Island in the late 1970s
and I was about to open the huge fish to try
and determine whether the black marlin that appeared
every spring off the Great Barrier Reef congregated
there to spawn or not.
Up until then I had been used to peering into the
insides of estuarine fish like bream, flathead and
whiting, checking their gonads for signs of development
- ripening eggs in ovaries and running milt in testes.
The sex of many familiar fish species can only be
determined by cutting them open to inspect their
reproductive organs. At the same time, it is possible
to roughly estimate the stage of their reproductive
cycle using a trained eye, followed by microscopic
examination of gonad tissue later in the laboratory.
MARLIN READY TO SPAWN
Armed with a large-bladed knife rather than a scalpel,
I sliced through the body wall of the grander marlin,
revealing the swollen pair of ovaries inside, which
when removed weighed a total of nearly 30kg! Clearly
visible throughout the yellow ovarian tissue were vast
numbers of what looked like small grains of sand.
These were in fact individual marlin eggs, each one
with the potential to one day turn into a fish the size
of its mother lying there before me.
The eggs were not in what is known as a hydrated
state, in which each one absorbs sea water and swells
to a much larger almost transparent globe prior to
being released out into the ocean to be fertilized by
one or more attendant male fish. However, I knew
that this process could happen within a few short
hours, and that this fish was indeed a ripe female
ready to spawn.
On the same trip, I was also able to cut open another
eight large female fish, all in spawning condition, as
well as three much smaller black marlin, ranging from
50 to 70kg. The latter were all males with very
well-developed testes containing copious
amounts of white sperm-bearing
liquid known as ‘milt’.Far left: Dr Julian Pepperell with one of the two ripe ovaries
he extracted from a spawning 500kg black marlin caught off
Cairns in the late 1970s. Together, the two ovaries weighed
30kg and would have contained millions of eggs.
Left: During his late 1970s trip to Cairns,
Dr Pepperell displays the testes of a male black
marlin caught during the October spawning season.
Freshly hatched from a tiny floating egg, this larval blue marlin
must eat ravenously to grow rapidly and so avoid being eaten
by the many other small predators in the open ocean.Even though they
grow to several
metres in length
within a few years,
all billfish – including
this baby sailfish
resting on the palm
of a hand – start life
as an egg the size of
a pinhead.bluewatermag.com.au 73